The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific quantum key distribution (QKD) systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33, while photon-entanglement is discussed in the same book on pages 53-92
Quantum key distribution (QKD) involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (i.e., 1 photon or less, on average, and typically 0.1 photon on average) optical signals or “qubits” transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. Thus, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal her presence.
In some QKD systems, entangled photon pairs are used instead of single photons. In 1991, Ekert proposed a QKD scheme based on entangled photons shared by Alice and Bob, (see A. K. Ekert. “Quantum cryptography based on Bell's theorem”, Phys. Rev. Lett., vol. 67 pp. 661-663, (1991) (hereinafter, “Ekert”) which article is incorporated by reference herein). In the Ekert 91 protocol, Alice and Bob also can check so-called Bell inequality to reveal the presence of the eavesdropper. Thus, the Ekert protocol provides an additional security level as compared to the BB84 protocol.
The article by J. Brendel, N. Gisin, W. Tittel, and H. Zbinden, entitled “Pulsed Energy-Time Entangled Twin-Photon Source for Quantum Communication”, PRL 82, 2594-2597 (22 Mar. 1999) (“Brendel”), which article is incorporated by reference herein, shows a QKD scheme that employs entangled photons. However, the Brendel system is subject to phase drifts and needs to be compensated in order to be of practical use in quantum cryptography. In addition, a commercially viable entanglement-based QKD system must be cost-effective and preferably built from of-the-shelf components. Currently, the 1550 nm wavelength is the most widely used wavelength for fiber-optics communication, so that use of 1550 nm components is preferred when constructing QKD systems, particularly those that meant to be integrated with standard telecommunications networks.